I don't think it's possible to "fantasize" about being raped. Fantasy connotes something that you dream about positively, i.e., you want it to happen. The thing that makes rape rape is the lack of consent. If you want to be raped then you probably consent to the act when it happens, negating the part of the crime that is the most important. Sounds like what you really want is spontaneous rough sex with someone you just met.